[theodorton]

Domains that have less frequent lookups so the chance of getting a cached response is lower.

[lloydatkinson]

Why aren't they just called infrequently used domains then?

[jasode]

>Why aren't they just called infrequently used domains then?

You could call them "infrequent" but "long-tail" is also a common description to convey a Power Law distribution: https://en.wikipedia.org/wiki/Long_tail

I think in this case about DNS caching, "long tail" is better than "infrequent". In the wikipedia graph, some of the domain lookups in yellow may be "frequent" (absolute sense) but simultaneously but much less popular (long tail) such that they don't stay in DNS lookup caches.